Tire Size Ratings
I was talking to a friend about the tires I want to purchase for my '70 LeMans Sport. This led to a heated discussion (and a bet) about tires and how their sizes are rated. My friend says a P235/50R15 tire is wider than a P235/70R15. I say both tires are the same width; the difference is the sidewall. My friend says he worked at a tire store. Who's right and how are tires rated?Steven PoindexterTecumseh, NE

2/6Passenger- car tires are designated by the section width in millimeters (P245 here), the aspect ratio in percent (65), a letter referencing the internal construction (R for radial), and the wheel diameter in inches the tire fits on (17); a service description number (105T) usually follows that and specifies the tire's load-carrying ability and speed rating.

Warning, Steven, the full, correct answer to this question is not an easy yes or no. The U.S. Tire and Rim Association Inc., in liaison with corresponding international tire and rim organizations, has established standard designation and interchangeability standards for tires, rims, and related components. When looking at today's standard passenger-car tire designation system, the key dimensions that are spelled out are the section width, the type of tire, the aspect ratio, and the wheel diameter the tire fits.

On your example P235/70R15 tire, the P designates it as a standard passenger-car tire, 235 is the section width, 70 is the aspect ratio, R means the tire is of radial construction, and 15 is the wheel diameter the tire mounts on.

A tire's section width (also known as cross-section width) is the distance from the widest point of the inner sidewall to the widest point of the outer sidewall, excluding any cosmetic ribs or raised letters, with no load on the tire. Section width is always expressed in millimeters. To convert millimeters to inches, multiply by 0.03937 (e.g., 235 x 0.03937 = 9.25195 inches).

In the real world, the actual tire section width varies depending on the width of the rim it is mounted on as well as its inflation pressure. Therefore, the tire industry has a designated standard rim width and inflation pressure for every tire size (generally 35 psi for standard-load P-metric tires). In other words, a 235 standard-load tire's section width will be 235 millimeters (or 9.25195 inches) only when mounted on the industry-designated standard rim width and inflated to 35 psi. If the tire is mounted on a narrower or wider wheel than specified, the section width becomes correspondingly narrower or wider; the rule of thumb is that every 0.5-inch change in rim width changes the section width approximately 0.2 inch.

A tire's sidewall aspect ratio or profile (50 and 70 in your examples) indicates the tire's section height as a percentage of the section width. Section height is the sidewall distance from the wheel mounting surface to the outer diameter of the tire. An aspect ratio of 50 means the section height is 50 percent of the section width (about 117.5 mm or 4.625975 inches for your tires). The higher the number, the taller the sidewall (and vice versa).

From the foregoing, it appears that tires with the same section width should have the same width regardless of aspect ratio. But there are several flies in the ointment. First, as stated previously, the designated section width is only achieved when the tire is mounted on the specified measuring rim width-but the designated measuring rim width changes not only by section width, but also according to the tire's aspect ratio. Lower-numbered aspect ratios for a given section width tend to have wider specified measuring rim widths, with the major change usually occurring between 60- and 55-series aspect ratios. Therefore, assuming you originally had 70-series tires on a given wheel and replaced them with 50-series tires of the same section width without changing wheel width, the new 50-series tires' effective section width is derived using a wider wheel and would actually be narrower than the 70-series section width when mounted on your original wheels.

Finally, there is no direct correlation between section width and tread width (the actual tire ground contact patch). Tires with identical section widths may have slightly different tread widths. Unfortunately, there is no industry standard for measuring tread width, which is further complicated because many modern tire profiles have rounded edges where the tread actually wraps around and merges with the sidewall without any sharp delineation between the two. Because a defined tread width measurement standard is lacking, many tire manufacturers no longer publish tread widths. Tire tread measurements, where still available, should only be used to compare different offerings from the same manufacturer, and (as is the case with camshaft advertised duration figures) should not be used to compare offerings from different manufacturers.

Finally, no major tire manufacturer that I am aware of offers a P235/50R15, although the 70-series version is still available. Much valuable information on tire design, designation, and characteristics can be found at the online tire store TireRack.com. As far as your bet goes, I guess we'll just have to call this one a draw-you're both in error.

T-Bolt 'Glass Doors
I'm building a '64 Ford Thunderbolt replica. I'd like to buy a set of fiberglass doors from AFX Components in Rose, New York, as listed on page 64 of Steve Magnante's great story on Ford T-bolts (HOT ROD, Nov. '09). I can't find them searching the web or in the Online Yellow Pages. Can you help with contact details?

Ray BaneyAlice Springs, Northern Territory, Australia

AFX Components appears to be gone with the wind, but Crites has a long list of Thunderbolt repop goodies (mechanical, body, and interior). It doesn't list fiberglass doors, though, as original Thunderbolts retained steel, '64 Fairlane doors. While fiberglass '64 Fairlane doors are available from Showcars Bodyparts Unlimited, they're full-race items that are set up for fixed Plexiglas windows with no provisions for the heavier roll-up glass windows you'd probably want on a street-driven car. Showcars says it can make a mold for heavy-duty fiberglass doors with roll-up window provisions if someone sends in a good pair of stock steel doors to copy.

540 Rat Head Bolts
Back in 2008 you built up a 540ci Chevy big-block. In the build you had trouble with the head bolts being too long, causing the head gaskets to not seal properly. Now I have a friend who is having the same problem. How did you finally solve it?

Fred KailerBroken Arrow, OK

I can tell you what worked for us, but your results may vary, what with all the different blocks, heads, and head gaskets out there, as well as the amount the block and/or heads may have been milled. Chevy Mark IV production blocks are pretty forgiving because the bottom of the head-bolt holes opens into a hollow water jacket. The HOT ROD 540 was built using an aftermarket Dart block that has blind head-bolt holes, meaning a slightly too-long bolt can bottom out in its hole. That's what happened when we installed Brodix Race-Rite oval-port heads on this block. Brodix says these heads are a direct bolt-on for stock cast-iron heads and should use a standard ARP head bolt kit for Rat motors with cast-iron heads (ARP PN 135-3601). We used that kit with our initially installed baseline GM iron oval-port heads with no problem. But our Race-Rites had been milled about 0.050 inch. That, combined with the blind head-bolt holes, shortened the bolt bosses just enough that the bolts located at each end of the middle row of head bolts underneath the valve cover bottomed out. They seemed to torque down normally but didn't achieve their full clamping load. The solution is to use shorter bolts or additional hardened washers under the bolt heads. In our case, we solved the problem by pirating the four longest bolts from a small-block Chevy head bolt kit (ARP PN 134-3601).

Brake Fluid Compared
Please clarify the differences and specific do's and don'ts regarding brake fluid. I've used DOT 5 (silicone), which conforms to FMVSS (Federal Motor Vehicle Safety Standard) No. 166. DOT 3 also meets the same specs. New cars come with DOT 3. I've heard rumors and arguments about the pros and cons of each type. I've also heard that DOT 5 should not be used in ABS systems.

Al RhyneFayetteville, GA

Actually, the factory fill on most U.S.-built cars since about 2006 has been DOT 4, and the relevant FMVSS standard is No. 116. To get the scoop on the great brake fluid debate, I consulted independent brake engineer Fred Callahan and other experts. Callahan says traditional polyalkylene glycol ether-based DOT 3 and DOT 4 brake fluids are hygroscopic, meaning they absorb water from the atmosphere under normal humidity levels. Excessive moisture in the system can drastically decrease the fluid's effective boiling point. If the fluid boils, goodbye brakes.

There will always be some water inclusion in a DOT 3- or 4-filled system: Once the calipers get hot, heating and cooling will eventually condense out the moisture from the air. However, the moisture ends up evenly dispersed in the brake fluid. Plus, today's DOT 3/4 formulations also contain corrosion inhibitors so the steel brake lines won't go away. Within reason, then, a street brake system remains reasonably effective even with a few percentage points of moisture in the system. But you can kiss your paint goodbye if any glycol-ether-based fluid is spilled on it.

To get around these perceived limitations and also achieve a higher boiling point, silicone-based DOT 5 brake fluids were introduced about 30 years ago. Silicone-based fluids are hydrophobic; they do not absorb water from the atmosphere. In theory, this means silicone fluid can maintain an acceptable boiling point for a much longer period of time. Silicone-based fluids won't screw up your paint, either.

So silicone sounds pretty good, right? Not so fast: During long-term, daily-driving usage, water will still get into the system, either from normal condensation from any small amount of air inevitably present in the system, washing the car, and/or changes in humidity. For sure, if the system was previously filled with conventional brake fluid, there will be some residual moisture present. The problem is that with silicone fluid, any in-system moisture remains separated, not evenly dispersed (kind of like how water and oil won't mix). This phase separation/water pooling characteristic actually makes silicone less tolerant of any in-system moisture than conventional brake fluids. The beads of moisture migrate through the brake lines and collect in the calipers. Caliper temperatures can exceed 200 degrees F under heavy braking conditions. At sea level, water boils at 212 degrees F, and the boiling point decreases with altitude (for example, at 5,000 feet, water boils at only 202 degrees). Oops.

And even uncontaminated silicone fluid has about two times greater compressibility than conventional brake fluids, especially at high temperatures. Silicone brake fluid is also adversely affected by atmospheric pressure; when a car with silicone fluid is driven at high altitudes, the fluid can expand significantly. Individually or collectively, this can result in a squishy, spongy brake pedal.

Combine a separate water pocket that forms in the system and then boils off with silicone's greater inherent compressibility: At a minimum the result is an unpredictable, soft pedal; at worst, unpredictable vapor lock and outright system failure. Murphy's Law rearing its ugly head again, this is bound to occur at the worst time-like going into a hard corner on the racetrack or coming down off a high mountain pass.

Wait, we're not done yet; there are several other problems with silicone: First, it is totally incompatible with glycol-based conventional brake fluids. Second, it's very hard to pour into the system without inducing air bubbles, making it hard to bleed. Third, silicone may cause the ethylene propylene rubber seals used in most conventional brake systems to swell up and even fail. Fourth, don't even think of using silicone in an ABS system: Its higher compressibility and increased amount of pedal travel throw off the assumptions programmed into the ABS computer and sensors.

To have any chance of getting silicone to work long-term on a non-ABS system, you must totally remove any trace of conventional brake fluid, moisture, and air from the system-not a realistic proposition for the average home mechanic. In theory, all the seals should also be replaced by silicone-compatible elastomers (Viton is one possibility). Consider, as well, revising the brake pedal linkage, power booster, and master cylinder to compensate for the spongy pedal feeling.

Is there a better solution than silicone for racers and hot rodders? You bet: The original DOT 3 and 4 specs were formulated years ago and specify minimum dry and wet boiling points. The actual dry boiling points of aftermarket, nominally DOT 3 or 4, professional-level racing brake fluids can be considerably higher, but you will usually pay a premium price for them. Depending on the specific fluid, the nominal wet boiling point may also increase significantly over parts store DOT 3/4 fluids, but despite a theoretically higher wet boiling point, the premium fluids all have a tendency to absorb moisture much more quickly. That's not critical for most racers as they frequently change their fluids but is an important consideration for a daily street driver. Niche racing fluids are made by AP Racing, Brembo, Wilwood, and others. Also note that some of these fluids should not be mixed with parts store fluids (check the specific manufacturer recommendations).

There is an alternative to the exotics for the budget-conscious racer or hot rodder: Ford Heavy Duty DOT 3 fluid (Ford PN C6AZ-19542-AA, Motorcraft PN PM1C), a widely available and inexpensive product that's popular among Sportsman-level racers because of its excellent dry boiling point (550 degrees F dry/290 degrees F wet). (If your Ford dealer has never heard of it, contact Ford of Upland.)

Any of these higher-end conventional brake fluids have a shorter-than-normal service life (about two years on the street versus up to five years for typical parts store brake fluid). Also, once you break the seal on any can of conventional glycol-based brake fluid, it starts absorbing moisture.

Although still somewhat rare in the U.S., for street-driven cars, there is also a new kid on the block: DOT 5.1. Although still hygroscopic like DOT 3/4 fluids, polyglycol ether-based, 5.1 street fluids have higher dry and wet boiling points than even silicone 5.0 brake fluid, but the DOT 5.1 fluids are still fully compatible with all rubber seal materials. They can usually be mixed with older DOT 3 and 4 fluids with no detrimental effects other than a reduction of ultimate boiling points. The 5.1 fluids are also said to be compatible with ABS systems. AP Racing is one source for 5.1 street-performance brake fluid (AP PN CP4510).

Bottom line: Leave the silicone fluid for the antique restoration guys or show car trailer queens who never really drive their cars anywhere.

Minimum Brake Fluid Boiling Points
The actual boiling point can vary higher by manufacturer (see text). In general, the dry (no moisture) boiling point is more critical for race cars; the wet boiling point (3.7 percent water by volume) is more critical for street-driven vehicles that inevitably experience moisture inclusion in the system.

SEA-LEVEL BOILING POINT

TYPE

DRY

WET

DOT 3

205° C (401° F)

140° C (284° F)

DOT 4

230° C (446° F)

155° C (311° F)

DOT 5

260° C (500° F)

180° C (356° F)

DOT 5.1

270° C (518° F)

190° C (374° F)

Resto Mopar Exhaust
Whose mufflers are on the undercarriage picture of the 'Cuda shown on page 96 of the Sept. '10 issue?

Bill DilworthVia email

The exhaust system shown in the photo was supplied by two niche Chrysler exhaust system specialists: TTI offers the headers and cross-pipe; Accurate Exhaust Products sells the high-perf AAR 'Cuda mufflers, side pipes, and exhaust tips.

3/6Accurate Exhaust Products specializes in reproduction exhaust systems for Chrysler, Dodge, and Plymouth muscle cars, as seen here on the 'Cuda equipped with a Moser MO875 rearend.

Gen VI Cam In Mark IV Big-Block Chevy
I have a Mark IV big-block Chevy with Edelbrock Performer RPM cylinder heads and SRP domed, forged pistons that make about 10.25:1 compression. I wanted to confirm that the hydraulic roller camshaft used in the new General Motors Performance ZZ427 could be used in the Mark IV block. The cam (GM PN 12366543) has 0.527-inch intake/0.544-inch exhaust lift and a duration of 224 degrees intake/234 degrees exhaust at 0.050. I believe this camshaft was used to increase the ZZ427 from 430 hp to 480 hp with 490 lb-ft.

Additionally, I would appreciate your recommendation for a fuel-injection system for this combination. The engine will be used in a '69 Camaro with a Tremec TKO-600 five-speed transmission and a four-point rear with 3.55:1 gears. The car will be street-driven.

Tom NickelBayport, NY

A Gen VI big-block hydraulic roller cam physically fits into the earlier Mark IV block, but (due to its different, stepped-nose configuration) it must be used with a corresponding Gen VI timing chain set. Also required are retrofit, guidebar-style, aftermarket hydraulic roller lifters and a hydraulic roller cam-compatible distributor-driven gear.

4/6Compared with a traditional big-block Chevy Mark IV cam (right), a late Gen VI-style cam (left) has a stepped nose with a reduced bolt circle. It must be used with a corresponding Gen VI timing set.

With any roller cam, a forward thrust-limiting solution is required to regulate camshaft endplay. The traditional method on a Mark IV block was to select-fit an available aftermarket camshaft thrust button or bearing pressed into the upper timing sprocket's center recess that limits forward travel by bottoming against the inside of a reinforced front timing cover. The Gen VI upper sprocket's center recess is a different, nonstandard size, so replicating this solution when using a Gen VI cam in a Mark IV block requires a fabricated custom stop. The other drawback to the traditional front button is that typical front cover gaskets (even those from the same manufacturer) vary slightly in thickness. If the front cover is removed and replaced, the endplay can also change.

Now there's a better way. When installed in a Gen VI block, excess forward travel on a Gen VI hydraulic roller cam is limited by a factory thrust plate. The reduced-diameter Gen VI cam nose indexes into the thrust plate's center hole, and the plate in turn bolts to the block's front cam boss. Although endplay still should be checked and verified (particularly when using thicker-than-stock aftermarket timing sets), it's not affected by different front gaskets.

It turns out that many old Mark IV blocks also have thrust-plate provisions. Back in the day, the plate was used on rare factory reverse-rotation, geardrive, flat-tappet cam applications. Westech Performance checked out an original factory high-performance block, and it turns out the thrust plate's bolt spacing (even though it performed a different function) is the same as on the late models. So assuming your Mark IV block is drilled with these two extra holes (as shown in the photos, approximately 2.394 inches on-center spacing), you can install a Gen VI cam and timing set in the Mark IV block using the late-GM Gen VI cam retainer. Sand the thrust-plate as needed to achieve the needed endplay (about 0.002 to 0.007 inch).

5/6Many Mark IV blocks (typically, high-performance, heavy-duty truck, or Marine applications) have two drilled boltholes on either side of the front cam bore. On this original production block at Westech Performance, the bolt spacing was the same as the Gen VI block's bolt spacing just rotated from the Gen VI's 12 and 6 o'clock position to 9 and 3 o'clock. If your early block has these holes, it can use the late-Gen VI cam retainer.

The one remaining drawback to this retrofit was the lack of a double-roller Gen VI timing chain. There was insufficient clearance underneath the factory eight-bolt Gen VI timing cover to clear the thicker double-roller chain, so until recently only single-roller Gen VI chains were available. But now Comp Cams sells a new Gen VI eight-bolt cover (PN 217) that clears its trick multikeyway billet Gen VI-style double-roller timing set (PN 7101). On the Mark IV block with a Gen VI-style cam, you would use the original 10-bolt Mark IV-style front cover with the now-available Gen VI double-roller chain.

Because the thrust-plate cam retention solution is a set-it-once-and-forget-it solution, with a Gen VI double-roller chain now available, anyone ordering a custom roller cam for an early Mark IV block with the two extra holes in the front cam journal boss might well consider ordering the custom cam machined with the late-style nose.

As for an EFI system recommendation, it depends on the level of sophistication you desire: For fuel control, do you want a throttle body (TBI) system, a direct-port batch-fire system, or a direct-port sequential-fire system? On the spark side, do you want to control the distributor with a computer, get rid of the distributor entirely and go to a coil-on-plug setup, or stay with a traditional standalone distributor? Do you want a simple hand-held programmer with limited options, a system that can be programmed using a PC computer, or a self-learning system? Is there a need to control other components besides fuel and spark (such as a late-model automatic trans or electric fans)? Once you decide on the type of system architecture that fits your needs, FAST, Holley, or other fuel-injection companies have products that will do the job. If it were me, I'd want at least a self-learning, PC-reprogrammable, batch-fire port system that can also control the distributor. One option is Holley's midlevel HP EFI setup.